With increasing risk of a nuclear accident or detonation, there is a critical need to develop effective medicinal countermeasures that can be delivered to victims after an incident to increase survival and prevent long-term effects in those that survive. Major causes of death following a nuclear incident include acute radiation syndromes (ARS) that destroy hematopoietic stem cells (H-ARS), gastrointestinal stem cells in the crypts (GI-ARS), or both. We have developed a unique regenerative peptide drug, TP508 (rusalatide acetate) that mitigates effects of radiation by restoring endothelial function and activating stem cells. Phase 1 and Phase 2 studies demonstrated that TP508 significantly increases survival and delays mortality when injected 24h after lethal doses of gamma radiation. In these studies, TP508 increased recovery of bone marrow stem cells and stem cells in intestinal and colonic crypts to maintain GI integrity. Our studies also showed that TP508 systemic injection activates stem cells in the dermis of irradiated animals adjacent to burns and accelerates healing and re-epithelization of the burned dermal area. These life-saving effects of TP508, therefore, appear to be mediated through the protection/activation of stem cells or endothelial cells that maintain the stem cell niche. Thus, in addition to mitigating effects of H-ARS and GI-ARS, TP508 may have widespread benefit to prevent delayed effects of radiation by stimulating tissue regeneration. To obtain FDA animal rule' licensure for TP508, we will complete steps necessary for approval employing the established regulatory pathway and animal models for H-ARS. Specific Aims for Phase 2b are designed to utilize animal studies that will allow us to predict the most efficacious dose of TP508 to be used in humans following a nuclear incident. We will establish optimal TP508 doses in two strains of mice and conduct GLP PK/PD studies in normal and irradiated mice to determine the required systemic bioavailability for maximum efficacy. Since most exposed individuals will receive supportive care following exposure, we will also determine how supportive care of our animals will affect TP508 mitigation of radiation-induced mortality. Although we have information regarding TP508 effects, additional experiments will be used to confirm the key cellular targets and modes of action. These studies will include experiments with normal and eNOS-/- mice using established assays to quantify TP508 effects on hematopoietic cell recovery, DNA repair, endothelial cell protection, and stem cell activation. For FDA approval, large animal studies predictive of human efficacy must also be completed. The large animal gold standard for H-ARS drugs is demonstration of increased survival of irradiated non-human primates (NHP). Therefore, we will conduct GLP PK/PD studies and a pilot efficacy study in NHP using the same formulation and route of administration that will be used in humans. Completion of this Phase 2B project will position TP508 for pivotal NHP blinded efficacy studies needed for licensure and potential stockpiling of the drug to save thousands of lives in the event of a nuclear disaster.